1. Field of the Invention
The present invention relates to an image reading apparatus, and more particularly, to an image reading apparatus which reads an image recorded onto a document which has been disposed on a plate-shaped transparent member such as a platen glass.
2. Description of the Related Art
Conventionally, regarding a copying machine, a technique has been known in which the size of a document disposed on a platen glass is detected, and on the basis of the detected result, the size of a paper on which an image recorded onto the document is copied, copying magnification or the like are automatically determined. The most common method of detecting the size of a document disposed on a platen glass is disclosed in JP-A No. 61-20936. In this method, one or more optical sensors for detecting the size of the document are provided beneath the platen glass, the optical sensors detect existence of the document at the position where the optical sensors are provided, and on the basis of the detected result by the optical sensors, the size of the document is determined.
However, in the aforementioned size detection method, it is necessary to provide optical sensors at various places beneath the platen glass so as to correspond to documents of various sizes. In reality, since a plurality of optical sensors are needed to cope with the documents of various sizes which can be disposed on the platen glass, there is a drawback in that the structure of the copying machine is complicated.
Further, the method of detecting the size of the document disposed on the platen glass is, more specifically, to detect whether the size of the document is any of the various sizes used as stylized sizes. In the aforementioned method of detection, the positions of the optical sensors are determined in accordance with the various sizes used as stylized sizes.
However, the sizes used as stylized sizes are different depending on the regions. For example, in Japan and in Europe, sizes such as B5, A4, B4 are used as stylized sizes, while in the United States and in Canada, sizes such as letter size, legal size, 11".times.17" are used as such. Accordingly, when the size of the document of the stylized size disposed on the platen glass is detected by the aforementioned method of detection, it is necessary to change the positions of a plurality of optical sensors for every region so as to correspond to the different sizes used as stylized sizes.
Moreover, after the image recorded onto the document is converted into a digitized image data, it is general that a copying machine (a so-called digital copying machine), which records (copies) the image onto a paper on the basis of the image data, or a facsimile terminal equipment includes a plurality of photoelectric transducing elements such as CCD which are arranged in a line, and that the photoelectric transducing elements read the image of the document by scanning the document. As a result, in this type of apparatus, it has been more widely used that the document is scanned by the photoelectric transducing elements before the image is read (a so-called pre-scan), and on the basis of the signals outputted from the photoelectric transducing elements, the size of the document is detected.
In the aforementioned method of detection, because the size of the document is also detected by the photoelectric transducing elements provided for reading the image, it is not necessary to provide a plurality of new sensors in accordance with the types of documents disposed on the platen glass. However, because it is necessary to pre-scan the document every time the document is disposed, there is a drawback in that the processing capacity (e.g., the copied number of sheets per unit of time or the like) of the apparatus does not improve.
As it is assumed that the document is disposed on the platen glass, it is general that the size of the document on the platen glass is detected when a platen cover is closed. However, because the reverse surface of the platen cover is normally white, in a case in which the background of the disposed document is white, i.e., the light reflectance of the document is similar to that of the reverse surface of the platen cover, it is difficult to detect optically the size of the document.
As a result, in order to improve reliability of the detection of the size of the document, it has been proposed that a surface whose light reflectance is low is formed on the reverse surface of the platen cover, or that the color of the reverse surface of the platen cover is not white (see JP-B No. 62-47026).
However, if the light reflectance of the reverse surface of the platen cover is remarkably lowered with respect to the light transmittance of the document whose background is white (e.g., the reverse surface of the platen cover is gray, black or a mirror finished surface), though detection of the size of the document becomes easy, the overall density of the background of the copied image becomes high, and a so-called set off occurs. The set off is especially conspicuous in a case in which a recorded image is copied onto a paper, such as a tracing paper, whose thickness is thin and whose light transmittance is relatively high.
Moreover, if the light reflectance of the reverse surface of the platen cover is substantially lowered with respect to the light transmittance of the document whose background is white, when the platen cover is closed and the document is read in the digital copying machine, the portion of the platen glass on which the document is not disposed is recognized as high density (e.g., black). Accordingly, when the image is copied onto the paper simply based on the image data obtained by reading the document, the portion of the copied image which corresponds to the portion at which the document is not disposed becomes solid black. Therefore, it is not preferable.
In order to avoid this, for example, after the pre-scan is effected and the size of the document is detected, the portion of the platen glass on which the document is not disposed is determined on the basis of the detected document size. When the document is read, the signal, which is outputted in accordance with the portion of the platen glass on which the document is not disposed, of the signals outputted from a photoelectric transducer needs to be masked electrically. Alternatively, it is necessary to change automatically input/output characteristic at the time of converting the signals outputted from the photoelectric transducer into the image data so that the density of the portion at which the document is not disposed is 0 (white) on the image data. A drawback arises in that the structure of the apparatus is complicated.
Instead of the above-described method, it has been effected that the reverse surface of the platen cover is a color whose light reflectance is similar to that of white and which is recognized as white by a monochrome copying machine (e.g., yellow). When the document is pre-scanned, an optical filter whose color is complementary to the color of the reverse surface of the platen cover (e.g., dark blue which is complementary to yellow) is inserted before a lens which images the light at the light-receiving portion of the photoelectric transducing element (as a result, the portion of the platen glass on which the document is not disposed is recognized as black), and when the image is read, the above-described optical filter is taken from before the lens. However, in this case, because it is necessary to provide a mechanism for moving the optical filter, there is a drawback in that the structure of the apparatus is complicated after all.